Wikijunior:Big Book of Fun Science Experiments/How A Bath Fizzer Works, and How To Make One
A bath fizzer or bath-bomb is a mixture of perfume or scent mixed an alkaline and an which reacts in water.
Look at the ingredients on baking powder: These usually just contain sodium bicarbonate which may be written as NaHCO3 but may also contain lemon juice or citric acid (H8O7). These are the alkaline and acid you need. Citric acid is the main ingredient of fizzy lemon or orange drink crystals and bath salts. in Europe, citric acid is often listed as as E330, bicarb is E500.
Some pharmacists stock boxes of pure crystalline sodium bicarbonate and refined citric acid crystals, but these things are cheaper and more easily sourced simply by reading the labels on various powder products sold in ordinary shops. The wanted product should be the main (ideally only) ingredient, but expect to find colour, flavour and texture products mixed.
The 'best' mixture will depend on the exact nature of the products you can find, but a good starting point is an equal mixture of both ingredients, which we call 1:1 ratio of 50%. You may have try different mixtures: 1:1, 1:2, 1:3, until the most lively reaction occurs, particularly with baking powder, which often contains rice flour.
Swirl the two dry powders together and then, with clean wet hands, squeeze them into a ball. The moisture on your hands will react with the outer layers, which will feel funny and form a sort of shell. Don't do this if you have cuts or grazes, because it can hurt. Once the ball has set, place it on a dry surface, wash your hands thoroughly and dry them. Getting just the right moisture to make the ball 'set' will require a bit of trial and error!
Now take some water that is as hot as is comfortable for a bath, drop the ball into it, and watch what happens!
Finally try adding some aromatic cooking powder such as dried lavender, vanilla or nutmeg. That's it!